Fertilization Levels Affect Growth of Red LeafLettuce and Chemical Characteristics ofRe-circulated Solution and Root Media

The growth of red leaf lettuce and changes in chemical properties of recycled nutrient solution and root media were investigated during five weeks of sub-irrigation cultivation. For the experiment, of seeds of lettuce (Lactuca sativa L.) 'Dduk Seom Jeok Chuk Myeon' were sown into 50-cell plug trays (cell volume 40 mL) filled with root substrate (coir-dust + expanded rice hull, 8:2, v/v) containing pre-planting fertilizers. Seed were germinated and allowed to grow to the two true leaf stage in growth chamber at 20 degrees C. Then, they were transplanted to the 170 mL plastic pots filled with same root substrate and relocated into a greenhouse. During the experiment, growth environment were maintained to 33/17 degrees C in the average day time/night temperatures, 40 to 60% in relative humidity and 150 to 200 mu mol.m(-2).s(-1) in photosynthetic photon flux. Crops were supplemented with 0, 100, 200 and 300 mg.L-1 fertilizer solution based on N content; the concentrations of all other essential nutrients were adjusted to provide equal ratios of these nutrients. During cultivation, samples of recycled nutrient solution were collected each week and analyzed for pH, electrical conductivity (EC) and nutrient concentrations. The growth measurement of above ground tissue and root medium analysis for chemical properties were conducted five weeks after transplantation. The increase in fertilization levels from 0 to 300 mg.L-1 resulted in a linear increase in fresh and dry weights. Of the 100, 200, and 300 mg.L-1 treatments, the 300 mg.L-1 treatment induced the highest pH and EC in the recycled nutrient solutions. The changes in the NO3-N, Ca, Mg, and SO4-S concentrations had a similar effect on the EC in the recycled nutrient solution. When upper, middle, and bottom parts of the root medium were analyzed five weeks after transplanting, the differences of ECs between the upper and bottom parts were greater as post- planting fertilizer concentrations became get lowered. The pH of the upper part of the root medium were 0.1-0.3 units higher than the pH of the bottom parts, suggesting that alkaline salts such as Ca and Mg accumulated in the upper parts of medium. The concentration of NO3-N in the upper root medium was 2.5 times as high as bottom part when supplemented with 200 mg.L-1 fertilizer, but the differences between the upper and bottom parts of the medium decreased with the 300 mg.L-1 fertilizer treatment. The increase in fertilization level resulted in a drastic decrease in PO4-P and SO4-S concentrations in the upper part of the root medium, but these nutrient concentrations did not significantly decrease in middle and bottom parts of the medium. These results indicate that nutrients accumulate in the upper part of root medium and the composition of the nutrient solution can vary when crops are cultivated using sub-irrigation techniques.